FIG. 1 shows a short-axis view of the heart at the level of the right atrium (RA) and left atrium (LA), in a plane generally parallel to the atrio-ventricular groove, and at the level of the aortic valve. This view is looking from caudal to cranial. FIG. 1 also shows the septum primum (SP), a flap-like structure, which normally covers the foramen ovale, an opening in the septum secundum (SS) of the heart. In utero, the foramen ovale serves as a physiologic conduit for right-to-left shunting of blood in the fetal heart. After birth, with the establishment of pulmonary circulation, the increased left atrial blood flow and pressure presses the septum primum (SP) against the walls of the septum secundum (SS), covering the foramen ovale and resulting in functional closure of the foramen ovale. This closure is usually followed by anatomical closure of the foramen ovale due to fusion of the septum primum (SP) to the septum secundum (SS).
Where anatomical closure of the foramen ovale does not occur, a patent foramen ovale (PFO) is created. A patent foramen ovale is a persistent, usually flap-like opening between the atrial septum primum (SP) and septum (SS) of a heart. A patent foramen ovale results when either partial or no fusion of the septum primum (SP) to the septum secundum (SS) occurs. In the case of partial fusion or no fusion, a persistent passageway (PFO track) exists between the septum primum (SP) and septum secundum (SS). This opening or passageway is typically parallel to the plane of the SP, and has a mouth which is generally oval in shape. FIG. 2 shows the opening of the PFO track viewed from an end of the track. Normally the opening is relatively tall, but quite narrow. The opening may be held closed due to the mean pressure in the LA being typically higher than in the RA. In this manner, the SP acts like a one-way valve, preventing fluid communication between the right and left atria through the PFO track. However, at times, the pressure may temporarily be higher in the RA, causing the PFO track to open up and allow some fluid to pass from the RA to the LA, as indicated in FIG. 3. Although the PFO track is often held closed, the endothelialized surfaces of the tissues forming the PFO track prevent the tissue from healing together and permanently closing the PFO track. As can be seen in FIG. 4, (a view from line “C-C” of FIG. 1), the SP is firmly attached to the SS around most of the perimeter of the Fossa Ovalis, but has an opening along one side. The SP is often connected, as shown, by two or more extensions of tissue along the sides of the PFO track.
Studies have shown that a relatively large percentage of adults have a patent foramen ovale (PFO). It is believed that embolism via a PFO may be a cause of a significant number of ischemic strokes, particularly in relatively young patients. It has been estimated that in 50% of cryptogenic strokes, a PFO is present. Blood clots which form in the venous circulation (e.g., the legs) can embolize, and may enter the arterial circulation via the PFO, subsequently entering the cerebral circulation, resulting in an embolic stroke. Blood clots may also form in the vicinity of the PFO, and embolize into the arterial circulation and into the cerebral circulation. Patients suffering a cryptogenic stroke or a transient ischemic attack (TIA) in the presence of a PFO often are considered for medical therapy to reduce the risk of a recurrent embolic event.
Pharmacological therapy often includes oral anticoagulants or antiplatelet agents. These therapies may lead to certain side effects, including hemorrhage. If pharmacologic therapy is unsuitable, open heart surgery may be employed to close a PFO with stitches, for example. Like other open surgical treatments, this surgery is highly invasive, risky, requires general anesthesia, and may result in lengthy recuperation.
Nonsurgical closure of PFOs is possible with umbrella-like devices developed for percutaneous closure of atrial septal defects (ASD) (a condition where there is not a well-developed septum primum (SP)). Many of these conventional devices used for ASDs, however, are technically complex, bulky, and difficult to deploy in a precise location. In addition, such devices may be difficult or impossible to retrieve and/or reposition should initial positioning not be satisfactory. Moreover, these devices are specially designed for ASDs and therefore may not be suitable to close and seal a PFO, particularly because the septum primum (SP) overlaps the septum secundum (SS).